Inside the Head of PyDanny

Hi, I'm Daniel Roy Greenfeld, and welcome to my blog. I write about Python, Django, and much more.

Python F-Strings Are Fun!

In python 3.6 we saw the adoption of Literal String Interpolation, or as they are known more commonly, f-strings. At first I was hesitant because... well... we've got multiple string tools already available:

one, two = 1, 2
_format = '{},{}'.format(one, two)
_percent = '%s,%s' % (one, two)
_concatenation = str(one) + ',' + str(two)
_join = ','.join((str(one),str(two)))
assert _format == _percent == _concatenation == _join

Adding f-strings to this mix didn't seem all that useful:

_fstring = f'{one},{two}'
assert _fstring == _format == _percent == _concatenation == _join

I was doubtful, but then I tried out f-strings on a non-trivial example. Now I'm hooked. Be it on local utility scripts or production code, I now instinctively gravitate toward their usage. In fact, f-strings are so useful that going back to earlier versions of Python now feels cumbersome.

The reason why I feel this way is that f-strings are concise but easy to understand. Thanks to intuitive expression evaluation I can compress more verbose commands into smaller lines of code that are more legible. Take a look:

_fstring = f'Total: {one + two}'  # Go f-string!
_format = 'Total: {}'.format(one + two)
_percent = 'Total: %s' % (one + two)
_concatenation = 'Total: ' + str(one + two)
assert _fstring == _format == _percent == _concatenation

The f-string example is four characters shorter than the closest alternative and is extremely easy to read. Indeed, put the f-string example in front of a non-programmer and they'll understand it fast. The same won't apply to the alternatives, odds are they'll ask what .format(), str(), and the % mean.

F-Strings Are Addictive

The conciseness and power of the intuitive expression evaluation can't be understated. On the surface f-strings seem like a small step forward for Python, but once I started using them I realized they were a huge step in codability for the language.

Now I'm hooked. I'm addicted to f-strings. When I step back to Python 3.5 or lower I feel like less of a Python coder. Yes, I have a problem with how much I lean on f-strings now, but I acknowledge my problem. I would go to therapy for it, but I believe I can manage the addiction for now.

Okay, enough joking, f-strings are awesome. Try them out.

A Utility Script Example

We just released Two Scoops of Django 1.11, which is written in LaTeX. Like most programming books we provide code examples in a repo for our readers. However, as we completey revised the code-highlighting, we had to rewrite our code extractor from the ground up. In a flurry of cowboy coding, I did so in thirty minutes using Python 3.6 while leaning on f-strings:

"""Two Scoops of Django 1.11 Code Extractor"""
import os
import shutil
from glob import glob

    print('Removed old code directory')
except FileNotFoundError:
print('Created new code directory')

STAR = '*'


    '\\begin{python}': '.py',
    '\\begin{badpython}': '.py',
    '\\begin{django}': '.html',
    '\\begin{baddjango}': '.html',
    '\\begin{plaintext}': '.txt',
    '\\begin{badplaintext}': '.txt',
    '\\begin{sql}': '.sql',
    '\\begin{makefile}': '',
    '\\begin{json}': '.json',
    '\\begin{bash}': '.txt',
    '\\begin{xml}': '.html',

LANGUAGE_END = {x.replace('begin', 'end'):y for x,y in LANGUAGE_START.items()}

def is_example(line, SWITCH):
    for key in SWITCH:
        if line.strip().startswith(key):
            return SWITCH[key]
    return None

def makefilename(chapter_num, in_example):
    return f'code/chapter_{chapter_num}_example_{str(example_num).zfill(2)}{in_example}'

if __name__ == '__main__':

    in_example = False
    starting = False
    for path in glob('chapters/*.tex'):
            chapter_num = int(path[9:11])
            chapter_num = path[9:11]
        except ValueError:
            if not path.lower().startswith('appendix'):
        example_num = 1
        with open(path) as f:
            lines = (x for x in f.readlines())
        for line in lines:
            if starting:
                # Crazy long string interpolation that should probably
                # be broken up but remains because it's easy for me to read
                filename =  f'code/chapter_{chapter_num}_example_{str(example_num).zfill(2)}{in_example}'
                dafile = open(filename, 'w')
                if in_example in ('.py', '.html'):
            if not in_example:
                mime = None
                in_example = is_example(line, LANGUAGE_START)
                if in_example:
                    starting = True
            mime = is_example(line, LANGUAGE_END)
            starting = False
            if mime:
                in_example = False
                example_num += 1

Published: 2017-04-27 00:05

Tags: twoscoops python django python python3


If you read this far, you might want to follow me on twitter or github and subscribe via email below (I'll email you new articles when I publish them).



Content Copyright © 2012-2018 Daniel Greenfeld. Proudly harnessed by Mountain, powered by Flask, and rendered by Frozen Flask, all of which take great advantage of Python.